I have a very thin surfboard; an inch or so, and it has FCS single fin plugs (the round ones with two boxes needed for one fin). The plugs are small and hence it seems that due to not much area to glue/laminate it to the boards foam etc. the rear box of the both side fins have come loose. I can see that when bending the fin, the finbox moves in the board a bit. It looks like it has separated from the bottom laminate, there is no water inside the board.

What's the best way to fix this, please? I have the FCS fusion boxes, but they are much bigger than the original boxes and I am a bit afraid of drilling the board so much and trying to install them, as especially I am worrying if I can make the bottom laminate strong enough etc.?
Another option I have been thinking about is to try to drill on the sides of the current plugs and and fill the surroundings of the plugs with resin and glass fillings, would this be viable at all, and what if the foam has broken a bit due to plugs' movement?

Any insights on this would be highly appreciated! This is my goto freestyle stapless boards and I love it, a real bummer this happened

I'm no expert, but have fixed a few FCS, and a few fake FCS boxs. The fusion ones are much better .
As the board is quite thin in that area, it is best to resin the new boxes to both the underside and topside of the board, this can be done after removing the old, earlier single circular inserts, and then routing out areas for the newer Fusion boxes, and using a screwdriver poke "tunnels" down to the underside of the upper surface, these will fill with resin and connect the new boxes to the underside of the deck, and also the base..
Try to keep the fin orientation correct, (you won't notice it after a while, but it will stop the "know alls" saying it's wrong!

Awesome, thanks for the info. How should I glass the bottom after installing the fusion boxes from the underside of the board? I mean how can I get a smooth, flat surface for the bottom of the board where the fusion boxes are installed? Doesn't the glassfiber which overlaps with the original board's glassfiber create a higher surface/bump to the board's bottom profile? Ie. if the original board had 3 layers of glass and I add 2 layers, then the new overlap section has total 5 layers? Or can I sand down this, but does it affect the durability?

I want to make this as strong as possible, but still wondering if it's better to just replace the single plugs and not use fusion boxes?

Hi, Will reply with full explanation later, when more time....
A new single or fusion box arrives with a 2mm wall around it's upper surface, this wall protrudes above the surface of the board, it's function is to stop resin flowing into the grub screw hole andfin hole, the resin flows out of the routed hole and onto the surface of the board, when hardened the resin and wall is sanded back to become a flat surface.

Jonysan, thanks very much! I appreciate your time and look forward to your explanation before doing anything. I have some knowledge on epoxy/resin and fixed a few dents, but this is on another level and therefore I very much listen more experienced guys. Thanks again.

The black area us the FCS single box, a new unused one has a 2mm wall around the top surface, remove the old box, try to save as much foam core as possible, make a few vertical tubes using a screwdriver , theses are to allow the resin to flow downwards and attach to the inside of the deck, mix up resin, add some filler powder to the mix , to thicken it out, but it's not really necessary.
lightly sand around the "plug hole" surface, and then leaving an inch or two of surface exposed, mask in a circle around the "plug hole".
Pour resin into the plug hole and down into the tubes, keep pouring until the resin is about 15 to 20mm from the top of the hole.

Insert the fin box, and check a few things,.... the fin slot is in the right orientation ! the top surface of the fin box is flush with the surface of the board, and the surface is level, ( the wall around the new box will be 2mm above the surface, and holding out the resin that should have overflowed out of the plug hole, having been displaced by the fin box,

This excess resin will flow onto the sanded, and exposed area around the plug hole,

Check everything is level, and allow to harden, you may find that some bubbles in the resin have risen to the surface and caused tiny blow holes, fill them later !

When fully hardened, using an orbital sander, or block, sand down the new resin until flush with the original surface, this will also sand down that plastic wall.

In the rough sketch, please note that the foam has been undercut beneath the original fibreglass surface, this will give a stronger and larger bonding area of the resin/surface bond.